In recent years, it has become common to mount a large package having more than 2000 terminal pins on a board (mainly motherboard) of a large-scale computing machine, such as a large-sized computer, because of an increase in the number of signals, and the size of such a large package has also increased to the extent of exceeding 40 mm×40 mm. Since the package is so large in size that it is equipped with stiffeners intended to prevent the warpage of the package as a whole.
FIG. 4 is a schematic cross-sectional view illustrating a conventional package board module.
In a package board module 10 illustrated in FIG. 4, an LSI chip 12 is mounted on the topside surface of a package board 11 by the fusion and fixation of solder balls 13. In addition, the package board module 10 is provided with a stiffener 14 fixed in contact with the topside surface of the package board 11 so as to surround the LSI chip 12. The stiffener 14 is intended to support the package board 11 and prevent the warpage of the package board 11. The package board module 10 has solder balls 15 on the underside surface of the package board 11 and the package board 11 is mounted on a motherboard, as will be explained hereinafter with reference to FIG. 5.
FIG. 5 is a cross-sectional configuration diagram illustrating a conventional package mounted module.
Here, components of the package board module same as those illustrated in FIG. 4 are assigned with the same reference numerals and will not be explained again.
In a package mounted module 20 illustrated in FIG. 5, the entirety of the package board module 10 illustrated in FIG. 4 is mounted on the topside surface of a motherboard 21 by the fusion and fixation of solder balls 15 on the underside surface of the package board 11 shown in FIG. 4. In addition, a stiffener 22 is disposed on the underside surface of the motherboard 21 in a position to interpose the motherboard 21 between the stiffer 22 and the package board 11. The stiffener 22 has an area wider than that of the package board 11, and the motherboard 21 and the stiffener 22 are fixed to each other with fastening components (screws 23 here) at plural locations on the periphery of the stiffener 22 falling outside the package board 11 in a schematic view of a stack of the package board 11, motherboard 21 and stiffener 22.
FIG. 6 is an explanatory diagram illustrating a problem with the conventional package mounted module shown in FIG. 5.
Conventionally, the stiffeners 14 and 22 have been formed using a single metal and, therefore, a difference in the temperature of an assembly between a solder melting point and a normal temperature or a temperature difference between the points in time of apparatus operation and apparatus shutdown causes curvature to take place between the LSI chip 12 and the package board 11 and between the package board 11 and the motherboard 21 due to a difference in the thermal expansion coefficient. Under normal conditions, the thermal expansion coefficients of the package board 11 and the motherboard 21 are as large as approximately 9 to 15 ppm/K and 17 to 22 ppm/K, respectively, as compared with the thermal expansion coefficient (approximately 4 ppm/K) of the LSI chip 12. Consequently, as illustrated in FIG. 6, the package mounted module 20 cools down to a normal temperature while being warped toward the motherboard 21 (convex toward the package board 11) after being subjected to solder bonding at a temperature equal to or higher than a solder melting point. Although the package board 11 and the motherboard 21 remain temporarily warped at this point, the package board 11 is forcibly corrected to be flat by the stiffener 14, and the motherboard 21 is also forcibly corrected to be flat by the stiffener 22 since the stiffeners 14 and 22 are flat, thereby causing stresses to arise in solder-bonded portions between the LSI chip 12 and the package board 11 and between the package board 11 and the motherboard 21. These stresses are one of the causes for significantly degrading the reliability of solder bonding and should preferably be reduced. This is especially true in recent years, as the use of lead has been prohibited by regulations on hazardous substances, and high melting point solder such as tin-silver-copper solder (melting point: 218° C.) has come into use in place of conventionally used eutectic tin-lead solder (melting point: 183° C.). As a result, the range of temperatures from a solder melting point to a normal temperature has widened, thereby causing more intense stress to occur.
Note here that Patent Document 1 proposes providing a bimetal structure to a board to allow the bimetal to operate in the reverse direction of the warpage of the board caused by a temperature change from a solder melting point to a normal temperature, thereby preventing the board from becoming warped.
However, this approach is intended to correct the warpage of the board by the board itself instead of correcting the warpage by the stiffeners 14 and 22, as has been explained with reference to FIGS. 4 to 6, and is not intended to prevent the occurrence of stress caused by forcibly correcting the warpage of the board.
Patent Document 1: Japanese Patent Laid-Open No. 2-116197